In changeable traffic signs up to this point, the light of one or more lamps has been divided up onto a number of dots of light that are arranged into symbols or alphabetic characters, and the change between displays has been brought about by turning the associated lamps on and off.
Since there have been successful efforts to produce light-emitting diodes (LEDs) with high light concentration, light strength and long service life in a number of colors or at least in all the established signal colors, there have been attempts to use the advantages of light-emitting diodes over ordinarily used incandescent lamps, such as emission of an oriented light beam, considerably longer service life and a very favorable energy ratio for colored light, in promotional and informational signs, and also for traffic signals. It was attempted, in particular, to replace the technologically expensive fiber optics in changeable traffic signs. The use in graphics-capable displays is also being promoted because, with appropriate wiring, each LED can be individually driven and therefore permits individually programmable representations and information.
Light-emitting diodes are distinguished from conventional incandescent lamps not only by their production of light by means of semiconductor technology, which generates a nearly monochromatic light, but also by integrated optical mechanisms for directing light which, on the one hand, improve the proportion of useful light, and, on the other, produce universal favorable light distribution characteristics in narrow and broad beam models, so that the LEDs can be used directly as a signal light without additional optical measures.
While no overriding regulations with regard to phototechnical characteristics exist for promotional and information signs, they have existed in the field of traffic engineering for a long time with, in particular, light color, brightness, light distribution and, above all, a very low phantom light (illusion of a turned-on signal light due to incident sunlight) being prescribed. Ordinary commercial models meet these requirements only in part, but are used nonetheless as long as customer-specific models of the LEDs are completely uneconomical and also cannot be implemented by some manufacturers for technological reasons.
If the LEDs are used directly in traffic engineering without additional optical measures, then light color, brightness and uniformity usually meet specifications, while the required light distribution can often be achieved only by the insertion of additional lenses. High phantom light is the main problem. The rounded end of the usually clear transparent LED element concentrates incident sunlight directly onto the highly reflective components in the interior of the LED, such as reflector and reflector rim, terminal lugs and contact points, from where it is reflected back. Because of the clear transparent LED element, the phantom light is relatively whitish and unfiltered and often appears brighter during an unfavorable sun position than the actual signal light.
It is becoming an established specification in traffic engineering that a sun position of 10.degree. vertically above the optical axis (usually the direction of maximum light emission) is assumed for the assessment of phantom light. At such angles, special measures must be taken under any conditions in order to limit the above-described effect.
Whereas, in signal transmitters, the signaling unit equipped with a number of LEDs in a fixed arrangement can be examined and improved in its totality with regard to phantom behavior, individual light-dot optics must be considered in changeable traffic signs, so that they can be combined in an arbitrary number and arrangement into symbols or alphabetic characters.
One known measure consists in placing a converging lens a suitable distance in front of a relatively wide-radiating LED (FIG. 8). Given sufficient distance from the LED, the sunlight incident at an angle is guided completely outside the LED and absorbed on housing surfaces. This arrangement, however, has the disadvantage of a large space requirement and is therefore not suited to universal application.
Another measure consists in placing horizontal lamellae (FIG. 9, top) or tubular sections (FIG. 9, middle) in front of the LED in order to deflect the sunlight; small, elongated sun blinds or chutes (FIG. 9, bottom) are also used, particularly for multiple LED light dots, and, in principle, these are also customary for signal transmitters. Here it is of particular disadvantage that these add-on elements must either be protected by a front pane from the effects of weather and dirt or frequently cleaned. They are used particularly for LED arrangements in a rectangular grid.
Another measure consists in the use of lenses or LED elements colored in the signal color (tinting). The sunlight must pass through the died component twice, wherein especially the extraneous color components of the light are filtered out, but the LED light only once, the coloring being as transparent to the actual signal color as possible. In this way, the sunlight is considerably attenuated, but the useable light is also reduced to a lesser extent. Not only is the reduced useable light strength, which must be compensated by a larger number of light dots, a disadvantage, but so is the phantom light in the signal color, which is viewed particularly critically in a number of applications.
Another disadvantage is the generally circularly symmetrical light radiation of light-emitting diodes, which has the effect that a large component of the light is unusable, radiated into irrelevant areas, unless optical measures are again taken.
Furthermore, ordinary commercial light-emitting diodes have radiation characteristics which generally do not agree with the required light distribution of the light dots. For this reason disproportionately more LEDs must often be used, barring additional optics, merely in order to have sufficient light in the low-light areas. In many cases, the required light distribution cannot be achieved without additional measures.
The problem of the invention is to develop a universal LED optical element for changeable traffic signs which can be used without a front pane and with a smooth outer surface and exhibits the advantages of LEDs, such as low power consumption, long service life and freedom from maintenance, but, on the other hand, exhibits no phantom light, which permits individually adaptable, in particular, oval light distributions without significant light losses, which can be adapted to different LED models, LED suppliers or radiation characteristics and permit a particularly small axial separation between adjacent optical elements.